Control of Resistive MHD Instabilities by External Current Drive

This chapter discusses how resistive Magneto hydrodynamic (MHD) stability can be actively controlled by external current drive, as it can be applied locally, that is in the vicinity of the resonant surface of the mode of interest, minimizing both the impact on the global discharge characteristics and the power requirements. The ability to generate a localized current with good control of the radial location is crucial to the success of the control schemes for resistive MHD instabilities. The chapter focuses on electron cyclotron current drive (ECCD), which possesses these properties and has been used most successfully for the control of resistive MHD instabilities using external current drive. It reviews the criteria for control of resistive MHD instabilities by localized ECCD. The chapter also discusses sawtooth control. Control of tearing modes can either be done in a ‘pre-emptive’ manner or by generating a helical current in an already existing island.